Bacterial cell structure
Encyclopedia
Bacteria
, despite their simplicity, contain a well-developed cell structure which is responsible for many of their unique biological properties. Many structural features are unique to bacteria
and are not found among archaea
or eukaryotes. Because of the simplicity of bacteria
relative to larger organisms and the ease with which they can be manipulated experimentally, the cell structure of bacteria
has been well studied, revealing many biochemical principles that have been subsequently applied to other organisms.
Perhaps the most elemental structural property of bacteria
is cell morphology
(shape). Typical examples include:
Cell shape is generally characteristic of a given bacterial species, but can vary depending on growth conditions. Some bacteria have complex life cycles involving the production of stalks and appendages (e.g. Caulobacter) and some produce elaborate structures bearing reproductive spores (e.g. Myxococcus, Streptomyces
). Bacteria
generally form distinctive cell morphologies when examined by light microscopy
and distinct colony morphologies when grown on Petri plates. These are often the first characteristics observed by a microbiologist
to determine the identity of an unknown bacterial culture.
is (with some exceptions) their small size. For example, Escherichia coli
cells, an "average" sized bacterium, are about 2 micrometre
s (μm) long and 0.5 μm in diameter, with a cell volume of 0.6 - 0.7 μm3. This corresponds to a wet mass of ca. 1 pg, assuming that the cell consists mostly of water. The dry mass of a single cell can be estimated as 20 % of the wet mass, amounting to 0.2 pg. About half of the dry mass of a bacterial cell consists of carbon, and also about half of it can be attributed to proteins. Therefore, a typical fully grown 1-liter culture of Escherichia coli
(at an optical density of 1.0, corresponding to ca. 109 cells/ml) yields ca. 1 g wet cell mass.
Small size is extremely important because it allows for a large surface area-to-volume ratio which allows for rapid uptake and intracellular distribution of nutrients and excretion of wastes. At low surface area-to-volume ratios the diffusion of nutrients and waste products across the bacterial cell membrane limits the rate at which microbial metabolism can occur, making the cell less evolutionarily fit. The reason for the existence of large cells is unknown, although it is speculated that the increased cell volume is used primarily for storage of excess nutrients.
As in other organisms, the bacterial cell wall
provides structural integrity to the cell. In prokaryotes, the primary function of the cell wall is to protect the cell from internal turgor pressure
caused by the much higher concentrations of proteins and other molecules inside the cell compared to its external environment. The bacterial cell wall differs from that of all other organisms by the presence of peptidoglycan
(poly-N-acetylglucosamine and N-acetylmuramic acid), which is located immediately outside of the cytoplasmic membrane. Peptidoglycan
is responsible for the rigidity of the bacterial cell wall and for the determination of cell shape. It is relatively porous and is not considered to be a permeability barrier for small substrates. While all bacterial cell walls (with a few exceptions e.g. extracellular parasites such as Mycoplasma
) contain peptidoglycan, not all cell walls have the same overall structures. Since the cell wall is required for bacterial survival, but is absent in eukaryotes, several antibiotics (penicillins and cephalosporins) stop bacterial infections by interfering with cell wall synthesis, while having no effects on human cells, because the peptidoglycans present in bacterial cell wall and absent in human cell wall.
There are two main types of bacterial cell walls, Gram positive and Gram negative, which are differentiated by their Gram staining
characteristics. For both Gram-positive and Gram-negative bacteria, particles of approximately 2 nm can pass through the peptidoglycan.
The matrix substances in the walls of Gram positive bacteria may be polysaccharides or teichoic acids. The latter are very widespread, but have been found only in Gram positive bacteria. There are two main types of teichoic acid: ribitol teichoic acids and glycerol teichoic acids. The latter one is more widespread. These acids are polymers of ribitol phosphate and glycerol phosphate, respectively, and only one type is found in the wall of any particular strain of bacteria. Teichoic acids form receptor sites for bacteriophages, and at least some of them are located on the surface of many Gram positive bacteria. this is very important
layer adjacent to the cytoplasmic membrane. This is responsible for the cell wall's inability to retain the crystal violet stain upon decolourisation with ethanol during Gram staining
. In addition to the peptidoglycan
layer, the Gram negative cell wall also contains an outer membrane composed by phospholipid
s and lipopolysaccharide
s, which face into the external environment. As the lipopolysaccharide
s are highly-charged, the Gram negative cell wall has an overall negative charge. The chemical structure of the outer membrane lipopolysaccharide
s is often unique to specific bacterial strains (i.e. sub-species) and is responsible for many of the antigen
ic properties of these strains.
such as acting as a permeability barrier for most molecules and serving as the location for the transport of molecules into the cell. In addition to these functions, prokaryotic membranes also function in energy conservation as the location about which a proton motive force is generated. Unlike eukaryotes, bacterial membranes (with some exceptions e.g. Mycoplasma
and methanotrophs) generally do not contain sterols. However, many microbes do contain structurally related compounds called hopanoids which likely fulfill the same function. Unlike eukaryotes, bacteria
can have a wide variety of fatty acid
s within their membranes. Along with typical saturated and unsaturated fatty acid
s, bacteria can contain fatty acids with additional methyl, hydroxy
or even cyclic groups. The relative proportions of these fatty acids can be modulated by the bacterium to maintain the optimum fluidity of the membrane (e.g. following temperature change).
As a phospholipid bilayer, the lipid portion of the outer membrane is impermeable to charged molecules. However, channels called porin
s are present in the outer membrane that allow for passive transport
of many ion
s, sugar
s and amino acid
s across the outer membrane. These molecules are therefore present in the periplasm, the region between the cytoplasmic and outer membranes. The periplasm contains the peptidoglycan layer and many proteins responsible for substrate binding or hydrolysis
and reception of extracellular signals. The periplasm it is thought to exist as a gel-like state rather than a liquid due to the high concentration of proteins and peptidoglycan
found within it. Because of its location between the cytoplasmic and outer membranes, signals received and substrates bound are available to be transported across the cytoplasmic membrane using transport and signalling proteins imbedded there.
Fimbrae are protein tubes that extend out from the outer membrane in many members of the Proteobacteria
. They are generally short in length and present in high numbers about the entire bacterial cell surface. Fimbrae usually function to facilitate the attachment of a bacterium to a surface (e.g. to form a biofilm
) or to other cells (e.g. animal cells during pathogenesis
)). A few organisms (e.g. Myxococcus) use fimbrae for motility to facilitate the assembly of multicellular structures such as fruiting bodies. Pili
are similar in structure to fimbrae but are much longer and present on the bacterial cell in low numbers. Pili
are involved in the process of bacterial conjugation
. Non-sex pili also aid bacteria in gripping surfaces.
An S-layer
(surface layer) is a cell surface protein layer found in many different bacteria
and in some archaea
, where it serves as the cell wall. All S-layer
s are made up of a two-dimensional array of proteins and have a crystalline appearance, the symmetry of which differs between species. The exact function of S-layer
s is unknown, but it has been suggested that they act as a partial permeability barrier for large substrates. For example, an S-layer
could conceivably keep extracellular proteins near the cell membrane by preventing their diffusion away from the cell. In some pathogenic species, an S-layer
may help to facilitate survival within the host by conferring protection against host defence mechanisms.
Many bacteria secrete extracellular polymers outside of their cell walls. These polymers are usually composed of polysaccharide
s and sometimes protein
. Capsules are relatively impermeable structures that cannot be stained with dyes such as India ink
. They are structures that help protect bacteria from phagocytosis and desiccation
. Slime layer
is involved in attachment of bacteria to other cells or inanimate surfaces to form biofilm
s. Slime layers can also be used as a food reserve for the cell.
Perhaps the most recognizable extracellular bacterial cell structures are silla
. Silla
are whip-like structures protruding from the bacterial cell wall and are responsible for bacterial motility
(i.e. movement). The arrangement of silla about the bacterial cell is unique to the species observed. Common forms include:
Silla
are complex structures that are composed of many different proteins. These include silla
, which makes up the whip-like tube and a protein
complex that spans the cell wall and cell membrane to form a motor that causes the silla
to rotate. This rotation is normally driven by proton motive force and are found in the body of the cell.
In comparison to eukaryotes, the intracellular features of the bacterial cell are extremely simple. Bacteria do not contain organelle
s in the same sense as eukaryote
s. Instead, the chromosome
and perhaps ribosomes are the only easily observable intracellular structures found in all bacteria
. They do exist, however, specialized groups of bacteria that contain more complex intracellular structures, some of which are discussed below.
Unlike eukaryotes, the bacterial chromosome
is not enclosed inside of a membrane-bound nucleus
but instead resides inside the bacterial cytoplasm
. This means that the transfer of cellular information through the processes of translation
, transcription
and DNA replication
all occur within the same compartment and can interact with other cytoplasmic structures, most notably ribosome
s. The bacterial chromosome is not packaged using histones to form chromatin
as in eukaryote
s but instead exists as a highly compact supercoiled structure, the precise nature of which remains unclear. Most bacterial chromosomes are circular
although some examples of linear chromosomes exist (e.g. Borrelia burgdorferi
). Along with chromosomal DNA, most bacteria also contain small independent pieces of DNA called plasmid
s that not essential for growth and often encode for traits that are advantageous but not essential to their bacterial host. Plasmid
s can be easily gained or lost by a bacterium and can be transferred between bacteria as a form of horizontal gene transfer
.
In most bacteria
the most numerous intracellular structure is the ribosome
, the site of protein synthesis
in all living organisms. All prokaryotes have 70S (where S=Svedberg
units) ribosomes while eukaryotes contain larger 80S ribosome
s in their cytosol
. The 70S ribosome
is made up of a 50S and 30S subunits. The 50S subunit contains the 23S and 5S rRNA while the 30S subunit contains the 16S rRNA. These rRNA molecules differ in size in eukaryotes and are complexed with a large number of ribosomal proteins, the number and type of which can vary slightly between organisms. While the ribosome
is the most commonly observed intracellular multiprotein complex in bacteria
other large complexes do occur and can sometimes be seen using microscopy
.
some microbes contain intracellular membranes in addition to (or as extensions of) their cytoplasmic membranes. An early idea was that bacteria might contain membrane folds termed mesosome
s, but these were later shown to be artifacts produced by the chemicals used to prepare the cells for electron microscopy. Examples of bacteria
containing intracellular membranes are phototroph
s, nitrifying bacteria
and methane
-oxidising bacteria. Intracellular membranes are also found in bacteria
belonging to the poorly studied Planctomycetes
group, although these membranes more closely resemble organellar membranes in eukaryotes and are currently of unknown function.
in prokaryotes. It was once thought that prokaryotic cells did not possess cytoskeleton
s, but recent advances in visualization technology and structure determination have shown that filaments indeed exist in these cells. In fact, homologues
for all major cytoskeletal proteins in eukaryotes have been found in prokaryotes. Cytoskeletal elements play essential roles in cell division
, protection, shape determination, and polarity determination in various prokaryotes.i
do not live in environments that contain large amounts of nutrients at all times. To accommodate these transient levels of nutrients bacteria
contain several different methods of nutrient storage in times of plenty for use in times of want. For example, many bacteria
store excess carbon in the form of polyhydroxyalkanoates
or glycogen
. Some microbes store soluble nutrients such as nitrate
in vacuole
s. Sulfur is most often stored as elemental (S0) granules which can be deposited either intra- or extracellularly. Sulfur granules are especially common in bacteria
that use hydrogen sulfide
as an electron source. Most of the above mentioned examples can be viewed using a microscope
and are surrounded by a thin nonunit membrane to separate them from the cytoplasm
.
ic bacteria that provides buoyancy
to these cells by decreasing their overall cell density
. Positive buoyancy is needed to keep the cells in the upper reaches of the water column, so that they can continue to perform photosynthesis
. They are made up of a shell of protein that has a highly hydrophobic inner surface, making it impermeable to water (and stopping water vapour from condensing inside) but permeable to most gases. Because the gas vesicle is a hollow cylinder, it is liable to collapse when the surrounding pressure
becomes too great. Natural selection has fine tuned the structure of the gas vesicle to maximise its resistance to buckling
, including an external strengthening protein, GvpC, rather like the green thread in a braided hosepipe. There is a simple relationship between the diameter of the gas vesicle and pressure at which it will collapse - the wider the gas vesicle the weaker it becomes. However, wider gas vesicles are more efficient, providing more buoyancy per unit of protein than narrow gas vesicles. Different species produce gas vesicle of different diameter, allowing them to colonise different depths of the water column (fast growing, highly competitive species with wide gas vesicles in the top most layers; slow growing, dark-adapted, species with strong narrow gas vesicles in the deeper layers). The diameter of the gas vesicle will also help determine which species survive in different bodies of water. Deep lakes that experience winter mixing, will exposes the cells to the hydrostatic pressure generated by the full water column. This will select for species with narrower, stronger gas vesicles.
The cell achieves its height in the water column by synthesising gas vesicles. As the cell rises up, it is able to increase its carbohydrate
load through increased photosynthesis. Too high and the cell will suffer photobleaching and possible death, however, the carbohydrate produced during photosynthesis increases the cell's density, causing it to sink. The daily cycle of carbohydrate build-up from photosynthesis and carbohydrate catabolism
during dark hours is enough to fine tune the cell's position in the water column, bring it up toward the surface when its carbohydrate levels are low and it needs to photosynthesis, and allowing it to sink away from the harmful UV radiation when the cell's carbohydrate levels have been replenished. An extreme excess of carbohydrate causes a significant change in the internal pressure of the cell, which causes the gas vesicles to buckle and collapse and the cell to sink out.
Carboxysome
s are intracellular structures found in many autotrophic bacteria
such as Cyanobacteria, Knallgasbacteria, Nitroso- and Nitrobacteria. They are proteinaceous structures resembling phage heads in their morphology
and contain the enzymes of carbon dioxide fixation in these organisms (especially ribulose bisphosphate carboxylase/oxygenase, RuBisCO, and carbonic anhydrase). It is thought that the high local concentration of the enzymes along with the fast conversion of bicarbonate to carbon dioxide by carbonic anhydrase allows faster and more efficient carbon dioxide fixation than possible inside the cytoplasm. Similar structures are known to harbor the coenzyme B12-containing glycerol dehydratase, the key enzyme of glycerol fermentation to 1,3-propanediol, in some Enterobacteriaceae (e. g. Salmonella).
Magnetosome
s are intracellular organelles found in magnetotactic bacteria
that allow them to sense and align themselves along a magnetic field (magnetotaxis). The ecological role of magnetotaxis is unknown but it is hypothesized to be involved in the determination of optimal oxygen concentrations. Magnetosome
s are composed of the mineral magnetite
or greigite and are surrounded by a lipid bilayer membrane. The morphology of magnetosome
s is species-specific.
Perhaps the most well known bacterial adaptation to stress is the formation of endospore
s. Endospore
s are bacterial survival structures that are highly resistant to many different types of chemical and environmental stresses and therefore enable the survival of bacteria
in environments that would be lethal for these cells in their normal vegetative form. It has been proposed that endospore
formation has allowed for the survival of some bacteria
for hundreds of millions of years (e.g. in salt crystals) although these publications have been questioned. Endospore
formation is limited to several genera of Gram-positive bacteria
such as Bacillus
and Clostridium
. It differs from reproductive spores in that only one spore is formed per cell resulting in no net gain in cell number upon endospore
germination. The location of an endospore
within a cell is species-specific and can be used to determine the identity of a bacterium.
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
, despite their simplicity, contain a well-developed cell structure which is responsible for many of their unique biological properties. Many structural features are unique to bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
and are not found among archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
or eukaryotes. Because of the simplicity of bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
relative to larger organisms and the ease with which they can be manipulated experimentally, the cell structure of bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
has been well studied, revealing many biochemical principles that have been subsequently applied to other organisms.
Cell morphology
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
is cell morphology
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
(shape). Typical examples include:
- coccusCoccusCoccus can be used to describe any bacterium that has a spherical shape. It is one of the three distinct types of bacteria shapes, the other two being bacillus and spirillum cells...
(spherical) - bacillusBacillusBacillus is a genus of Gram-positive, rod-shaped bacteria and a member of the division Firmicutes. Bacillus species can be obligate aerobes or facultative anaerobes, and test positive for the enzyme catalase. Ubiquitous in nature, Bacillus includes both free-living and pathogenic species...
(rod-like) - spirillumSpirillumSpirillum in microbiology refers to a bacterium with a cell body that twists like a spiral. It is the third distinct bacterial cell shape type besides coccus and bacillus cells.-Taxonomic:...
(spiral) - filamentousFilamentationFilamentation is the anomalous growth of certain bacteria, such as E. coli, in which cells continue to elongate but do not divide . Bacterial filamentation is often observed as a result of bacteria responding to various stresses, including DNA damage or inhibition of replication...
Cell shape is generally characteristic of a given bacterial species, but can vary depending on growth conditions. Some bacteria have complex life cycles involving the production of stalks and appendages (e.g. Caulobacter) and some produce elaborate structures bearing reproductive spores (e.g. Myxococcus, Streptomyces
Streptomyces
Streptomyces is the largest genus of Actinobacteria and the type genus of the family Streptomycetaceae. Over 500 species of Streptomyces bacteria have been described. As with the other Actinobacteria, streptomycetes are gram-positive, and have genomes with high guanine and cytosine content...
). Bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
generally form distinctive cell morphologies when examined by light microscopy
Microscopy
Microscopy is the technical field of using microscopes to view samples and objects that cannot be seen with the unaided eye...
and distinct colony morphologies when grown on Petri plates. These are often the first characteristics observed by a microbiologist
Microbiologist
A microbiologist is a scientist who works in the field of microbiology. Microbiologists study organisms called microbes. Microbes can take the form of bacteria, viruses, fungi, and protists...
to determine the identity of an unknown bacterial culture.
The importance of cell size
Perhaps the most obvious structural characteristic of bacteriaBacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
is (with some exceptions) their small size. For example, Escherichia coli
Escherichia coli
Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...
cells, an "average" sized bacterium, are about 2 micrometre
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
s (μm) long and 0.5 μm in diameter, with a cell volume of 0.6 - 0.7 μm3. This corresponds to a wet mass of ca. 1 pg, assuming that the cell consists mostly of water. The dry mass of a single cell can be estimated as 20 % of the wet mass, amounting to 0.2 pg. About half of the dry mass of a bacterial cell consists of carbon, and also about half of it can be attributed to proteins. Therefore, a typical fully grown 1-liter culture of Escherichia coli
Escherichia coli
Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...
(at an optical density of 1.0, corresponding to ca. 109 cells/ml) yields ca. 1 g wet cell mass.
Small size is extremely important because it allows for a large surface area-to-volume ratio which allows for rapid uptake and intracellular distribution of nutrients and excretion of wastes. At low surface area-to-volume ratios the diffusion of nutrients and waste products across the bacterial cell membrane limits the rate at which microbial metabolism can occur, making the cell less evolutionarily fit. The reason for the existence of large cells is unknown, although it is speculated that the increased cell volume is used primarily for storage of excess nutrients.
As in other organisms, the bacterial cell wall
Cell wall
The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...
provides structural integrity to the cell. In prokaryotes, the primary function of the cell wall is to protect the cell from internal turgor pressure
Turgor pressure
Turgor Pressure or turgidity is the main pressure of the cell contents against the cell wall in plant cells and bacteria cells, determined by the water content of the vacuole, resulting from osmotic pressure, i.e...
caused by the much higher concentrations of proteins and other molecules inside the cell compared to its external environment. The bacterial cell wall differs from that of all other organisms by the presence of peptidoglycan
Peptidoglycan
Peptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria , forming the cell wall. The sugar component consists of alternating residues of β- linked N-acetylglucosamine and N-acetylmuramic acid...
(poly-N-acetylglucosamine and N-acetylmuramic acid), which is located immediately outside of the cytoplasmic membrane. Peptidoglycan
Peptidoglycan
Peptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria , forming the cell wall. The sugar component consists of alternating residues of β- linked N-acetylglucosamine and N-acetylmuramic acid...
is responsible for the rigidity of the bacterial cell wall and for the determination of cell shape. It is relatively porous and is not considered to be a permeability barrier for small substrates. While all bacterial cell walls (with a few exceptions e.g. extracellular parasites such as Mycoplasma
Mycoplasma
Mycoplasma refers to a genus of bacteria that lack a cell wall. Without a cell wall, they are unaffected by many common antibiotics such as penicillin or other beta-lactam antibiotics that target cell wall synthesis. They can be parasitic or saprotrophic. Several species are pathogenic in humans,...
) contain peptidoglycan, not all cell walls have the same overall structures. Since the cell wall is required for bacterial survival, but is absent in eukaryotes, several antibiotics (penicillins and cephalosporins) stop bacterial infections by interfering with cell wall synthesis, while having no effects on human cells, because the peptidoglycans present in bacterial cell wall and absent in human cell wall.
There are two main types of bacterial cell walls, Gram positive and Gram negative, which are differentiated by their Gram staining
Gram staining
Gram staining is a method of differentiating bacterial species into two large groups ....
characteristics. For both Gram-positive and Gram-negative bacteria, particles of approximately 2 nm can pass through the peptidoglycan.
The Gram positive cell wall
Peptidoglycans(mucopeptides, glycopeptides, mureins) are the structural elements of almost all bacterial cell walls. They constitute almost 95% of the cell wall in some Gram positive bacteria and as little as 5-10% of the cell wall in Gram negative bacteria. Peptidoglycans are made up of a polysaccharide backbone consisting of alternating N-acetylmuramic acid(NAM) and N-acetylglucosamine(NAG) residues in equal amounts. The cell wall of some Gram positive bacteria is completely dissolved by lysozyme, as this enzyme attacks the bonds between GA and MA. In other Gram positive bacteria, e.g. Staphylococcus aureus, the walls are resistant to the action of lysozyme. They have O-acetyl groups on carbon-6 of some MA residues.The matrix substances in the walls of Gram positive bacteria may be polysaccharides or teichoic acids. The latter are very widespread, but have been found only in Gram positive bacteria. There are two main types of teichoic acid: ribitol teichoic acids and glycerol teichoic acids. The latter one is more widespread. These acids are polymers of ribitol phosphate and glycerol phosphate, respectively, and only one type is found in the wall of any particular strain of bacteria. Teichoic acids form receptor sites for bacteriophages, and at least some of them are located on the surface of many Gram positive bacteria. this is very important
The Gram negative cell wall
Unlike the Gram positive cell wall, the Gram negative cell wall contains a thin peptidoglycanPeptidoglycan
Peptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria , forming the cell wall. The sugar component consists of alternating residues of β- linked N-acetylglucosamine and N-acetylmuramic acid...
layer adjacent to the cytoplasmic membrane. This is responsible for the cell wall's inability to retain the crystal violet stain upon decolourisation with ethanol during Gram staining
Gram staining
Gram staining is a method of differentiating bacterial species into two large groups ....
. In addition to the peptidoglycan
Peptidoglycan
Peptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria , forming the cell wall. The sugar component consists of alternating residues of β- linked N-acetylglucosamine and N-acetylmuramic acid...
layer, the Gram negative cell wall also contains an outer membrane composed by phospholipid
Phospholipid
Phospholipids are a class of lipids that are a major component of all cell membranes as they can form lipid bilayers. Most phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline; one exception to this rule is sphingomyelin, which is derived from...
s and lipopolysaccharide
Lipopolysaccharide
Lipopolysaccharides , also known as lipoglycans, are large molecules consisting of a lipid and a polysaccharide joined by a covalent bond; they are found in the outer membrane of Gram-negative bacteria, act as endotoxins and elicit strong immune responses in animals.-Functions:LPS is the major...
s, which face into the external environment. As the lipopolysaccharide
Lipopolysaccharide
Lipopolysaccharides , also known as lipoglycans, are large molecules consisting of a lipid and a polysaccharide joined by a covalent bond; they are found in the outer membrane of Gram-negative bacteria, act as endotoxins and elicit strong immune responses in animals.-Functions:LPS is the major...
s are highly-charged, the Gram negative cell wall has an overall negative charge. The chemical structure of the outer membrane lipopolysaccharide
Lipopolysaccharide
Lipopolysaccharides , also known as lipoglycans, are large molecules consisting of a lipid and a polysaccharide joined by a covalent bond; they are found in the outer membrane of Gram-negative bacteria, act as endotoxins and elicit strong immune responses in animals.-Functions:LPS is the major...
s is often unique to specific bacterial strains (i.e. sub-species) and is responsible for many of the antigen
Antigen
An antigen is a foreign molecule that, when introduced into the body, triggers the production of an antibody by the immune system. The immune system will then kill or neutralize the antigen that is recognized as a foreign and potentially harmful invader. These invaders can be molecules such as...
ic properties of these strains.
The bacterial cytoplasmic membrane
The bacterial cytoplasmic membrane is composed of a phospholipid bilayer and thus has all of the general functions of a cell membraneCell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
such as acting as a permeability barrier for most molecules and serving as the location for the transport of molecules into the cell. In addition to these functions, prokaryotic membranes also function in energy conservation as the location about which a proton motive force is generated. Unlike eukaryotes, bacterial membranes (with some exceptions e.g. Mycoplasma
Mycoplasma
Mycoplasma refers to a genus of bacteria that lack a cell wall. Without a cell wall, they are unaffected by many common antibiotics such as penicillin or other beta-lactam antibiotics that target cell wall synthesis. They can be parasitic or saprotrophic. Several species are pathogenic in humans,...
and methanotrophs) generally do not contain sterols. However, many microbes do contain structurally related compounds called hopanoids which likely fulfill the same function. Unlike eukaryotes, bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
can have a wide variety of fatty acid
Fatty acid
In chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...
s within their membranes. Along with typical saturated and unsaturated fatty acid
Fatty acid
In chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...
s, bacteria can contain fatty acids with additional methyl, hydroxy
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
or even cyclic groups. The relative proportions of these fatty acids can be modulated by the bacterium to maintain the optimum fluidity of the membrane (e.g. following temperature change).
As a phospholipid bilayer, the lipid portion of the outer membrane is impermeable to charged molecules. However, channels called porin
Porin (protein)
Porins are beta barrel proteins that cross a cellular membrane and act as a pore through which molecules can diffuse. Unlike other membrane transport proteins, porins are large enough to allow passive diffusion, i.e., they act as channels that are specific to different types of molecules...
s are present in the outer membrane that allow for passive transport
Passive transport
Passive transport means moving biochemicals and other atomic or molecular substances across membranes. Unlike active transport, this process does not involve chemical energy, because, unlike in an active transport, the transport across membrane is always coupled with the growth of entropy of the...
of many ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s, sugar
Sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...
s and amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
s across the outer membrane. These molecules are therefore present in the periplasm, the region between the cytoplasmic and outer membranes. The periplasm contains the peptidoglycan layer and many proteins responsible for substrate binding or hydrolysis
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
and reception of extracellular signals. The periplasm it is thought to exist as a gel-like state rather than a liquid due to the high concentration of proteins and peptidoglycan
Peptidoglycan
Peptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria , forming the cell wall. The sugar component consists of alternating residues of β- linked N-acetylglucosamine and N-acetylmuramic acid...
found within it. Because of its location between the cytoplasmic and outer membranes, signals received and substrates bound are available to be transported across the cytoplasmic membrane using transport and signalling proteins imbedded there.
Fimbrae and Pili
Main article: PilusPilus
right|thumb|350px|Schematic drawing of bacterial conjugation. 1- Donor cell produces pilus. 2- Pilus attaches to recipient cell, brings the two cells together. 3- The mobile plasmid is nicked and a single strand of DNA is then transferred to the recipient cell...
Fimbrae are protein tubes that extend out from the outer membrane in many members of the Proteobacteria
Proteobacteria
The Proteobacteria are a major group of bacteria. They include a wide variety of pathogens, such as Escherichia, Salmonella, Vibrio, Helicobacter, and many other notable genera....
. They are generally short in length and present in high numbers about the entire bacterial cell surface. Fimbrae usually function to facilitate the attachment of a bacterium to a surface (e.g. to form a biofilm
Biofilm
A biofilm is an aggregate of microorganisms in which cells adhere to each other on a surface. These adherent cells are frequently embedded within a self-produced matrix of extracellular polymeric substance...
) or to other cells (e.g. animal cells during pathogenesis
Pathogenesis
The pathogenesis of a disease is the mechanism by which the disease is caused. The term can also be used to describe the origin and development of the disease and whether it is acute, chronic or recurrent...
)). A few organisms (e.g. Myxococcus) use fimbrae for motility to facilitate the assembly of multicellular structures such as fruiting bodies. Pili
Pilus
right|thumb|350px|Schematic drawing of bacterial conjugation. 1- Donor cell produces pilus. 2- Pilus attaches to recipient cell, brings the two cells together. 3- The mobile plasmid is nicked and a single strand of DNA is then transferred to the recipient cell...
are similar in structure to fimbrae but are much longer and present on the bacterial cell in low numbers. Pili
Pilus
right|thumb|350px|Schematic drawing of bacterial conjugation. 1- Donor cell produces pilus. 2- Pilus attaches to recipient cell, brings the two cells together. 3- The mobile plasmid is nicked and a single strand of DNA is then transferred to the recipient cell...
are involved in the process of bacterial conjugation
Bacterial conjugation
Bacterial conjugation is the transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells...
. Non-sex pili also aid bacteria in gripping surfaces.
S-layers
Main article: S-layerS-layer
An S-layer is a part of the cell envelope commonly found in bacteria, as well as among archaea.It consists of a monomolecular layer composed of identical proteins or glycoproteins. This two-dimensional structure is built via self-assembly and encloses the whole cell surface. Thus, the S-layer...
An S-layer
S-layer
An S-layer is a part of the cell envelope commonly found in bacteria, as well as among archaea.It consists of a monomolecular layer composed of identical proteins or glycoproteins. This two-dimensional structure is built via self-assembly and encloses the whole cell surface. Thus, the S-layer...
(surface layer) is a cell surface protein layer found in many different bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
and in some archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
, where it serves as the cell wall. All S-layer
S-layer
An S-layer is a part of the cell envelope commonly found in bacteria, as well as among archaea.It consists of a monomolecular layer composed of identical proteins or glycoproteins. This two-dimensional structure is built via self-assembly and encloses the whole cell surface. Thus, the S-layer...
s are made up of a two-dimensional array of proteins and have a crystalline appearance, the symmetry of which differs between species. The exact function of S-layer
S-layer
An S-layer is a part of the cell envelope commonly found in bacteria, as well as among archaea.It consists of a monomolecular layer composed of identical proteins or glycoproteins. This two-dimensional structure is built via self-assembly and encloses the whole cell surface. Thus, the S-layer...
s is unknown, but it has been suggested that they act as a partial permeability barrier for large substrates. For example, an S-layer
S-layer
An S-layer is a part of the cell envelope commonly found in bacteria, as well as among archaea.It consists of a monomolecular layer composed of identical proteins or glycoproteins. This two-dimensional structure is built via self-assembly and encloses the whole cell surface. Thus, the S-layer...
could conceivably keep extracellular proteins near the cell membrane by preventing their diffusion away from the cell. In some pathogenic species, an S-layer
S-layer
An S-layer is a part of the cell envelope commonly found in bacteria, as well as among archaea.It consists of a monomolecular layer composed of identical proteins or glycoproteins. This two-dimensional structure is built via self-assembly and encloses the whole cell surface. Thus, the S-layer...
may help to facilitate survival within the host by conferring protection against host defence mechanisms.
Capsules and Slime Layers
Main article: Slime layerSlime layer
A slime layer in bacteria is an easily removed, diffuse, unorganized layer of extracellular material that surrounds bacteria cells. Specifically, this consists mostly of exopolysaccharides, glycoproteins, and glycolipids....
Many bacteria secrete extracellular polymers outside of their cell walls. These polymers are usually composed of polysaccharide
Polysaccharide
Polysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...
s and sometimes protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
. Capsules are relatively impermeable structures that cannot be stained with dyes such as India ink
India ink
India ink is a simple black ink once widely used for writing and printing and now more commonly used for drawing, especially when inking comic books and comic strips.-Composition:...
. They are structures that help protect bacteria from phagocytosis and desiccation
Desiccation
Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic substance that induces or sustains such a state in its local vicinity in a moderately sealed container.-Science:...
. Slime layer
Slime layer
A slime layer in bacteria is an easily removed, diffuse, unorganized layer of extracellular material that surrounds bacteria cells. Specifically, this consists mostly of exopolysaccharides, glycoproteins, and glycolipids....
is involved in attachment of bacteria to other cells or inanimate surfaces to form biofilm
Biofilm
A biofilm is an aggregate of microorganisms in which cells adhere to each other on a surface. These adherent cells are frequently embedded within a self-produced matrix of extracellular polymeric substance...
s. Slime layers can also be used as a food reserve for the cell.
Silla
Main article: SillaSilla
Silla was one of the Three Kingdoms of Korea, and one of the longest sustained dynasties in...
Perhaps the most recognizable extracellular bacterial cell structures are silla
Silla
Silla was one of the Three Kingdoms of Korea, and one of the longest sustained dynasties in...
. Silla
Silla
Silla was one of the Three Kingdoms of Korea, and one of the longest sustained dynasties in...
are whip-like structures protruding from the bacterial cell wall and are responsible for bacterial motility
Motility
Motility is a biological term which refers to the ability to move spontaneously and actively, consuming energy in the process. Most animals are motile but the term applies to single-celled and simple multicellular organisms, as well as to some mechanisms of fluid flow in multicellular organs, in...
(i.e. movement). The arrangement of silla about the bacterial cell is unique to the species observed. Common forms include:
- Peritrichous - Multiple silla found at several locations about the cell
- Polar - Single silla found at one of the cell poles
- Lophotrichous - A tuft of silla found at one cell pole
Silla
Silla
Silla was one of the Three Kingdoms of Korea, and one of the longest sustained dynasties in...
are complex structures that are composed of many different proteins. These include silla
Silla
Silla was one of the Three Kingdoms of Korea, and one of the longest sustained dynasties in...
, which makes up the whip-like tube and a protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
complex that spans the cell wall and cell membrane to form a motor that causes the silla
Silla
Silla was one of the Three Kingdoms of Korea, and one of the longest sustained dynasties in...
to rotate. This rotation is normally driven by proton motive force and are found in the body of the cell.
Intracellular bacterial cell structures
Organelle
In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....
s in the same sense as eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
s. Instead, the chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
and perhaps ribosomes are the only easily observable intracellular structures found in all bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
. They do exist, however, specialized groups of bacteria that contain more complex intracellular structures, some of which are discussed below.
The bacterial chromosome and plasmids
Main article: PlasmidPlasmid
In microbiology and genetics, a plasmid is a DNA molecule that is separate from, and can replicate independently of, the chromosomal DNA. They are double-stranded and, in many cases, circular...
Unlike eukaryotes, the bacterial chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
is not enclosed inside of a membrane-bound nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
but instead resides inside the bacterial cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
. This means that the transfer of cellular information through the processes of translation
Translation
Translation is the communication of the meaning of a source-language text by means of an equivalent target-language text. Whereas interpreting undoubtedly antedates writing, translation began only after the appearance of written literature; there exist partial translations of the Sumerian Epic of...
, transcription
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
and DNA replication
DNA replication
DNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance. The process starts with one double-stranded DNA molecule and produces two identical copies of the molecule...
all occur within the same compartment and can interact with other cytoplasmic structures, most notably ribosome
Ribosome
A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
s. The bacterial chromosome is not packaged using histones to form chromatin
Chromatin
Chromatin is the combination of DNA and proteins that make up the contents of the nucleus of a cell. The primary functions of chromatin are; to package DNA into a smaller volume to fit in the cell, to strengthen the DNA to allow mitosis and meiosis and prevent DNA damage, and to control gene...
as in eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
s but instead exists as a highly compact supercoiled structure, the precise nature of which remains unclear. Most bacterial chromosomes are circular
Circular bacterial chromosome
Circular bacterial chromosome are the bacterial chromosomes contained in a circular DNA molecule. Unlike the linear DNA of vertebrates, typical bacterial chromosomes contain circular DNA....
although some examples of linear chromosomes exist (e.g. Borrelia burgdorferi
Borrelia burgdorferi
Borrelia burgdorferi is a species of Gram negative bacteria of the spirochete class of the genus Borrelia. B. burgdorferi is predominant in North America, but also exists in Europe, and is the agent of Lyme disease....
). Along with chromosomal DNA, most bacteria also contain small independent pieces of DNA called plasmid
Plasmid
In microbiology and genetics, a plasmid is a DNA molecule that is separate from, and can replicate independently of, the chromosomal DNA. They are double-stranded and, in many cases, circular...
s that not essential for growth and often encode for traits that are advantageous but not essential to their bacterial host. Plasmid
Plasmid
In microbiology and genetics, a plasmid is a DNA molecule that is separate from, and can replicate independently of, the chromosomal DNA. They are double-stranded and, in many cases, circular...
s can be easily gained or lost by a bacterium and can be transferred between bacteria as a form of horizontal gene transfer
Horizontal gene transfer
Horizontal gene transfer , also lateral gene transfer , is any process in which an organism incorporates genetic material from another organism without being the offspring of that organism...
.
Ribosomes and other multiprotein complexes
Main article: RibosomeRibosome
A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
In most bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
the most numerous intracellular structure is the ribosome
Ribosome
A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
, the site of protein synthesis
Protein biosynthesis
Protein biosynthesis is the process in which cells build or manufacture proteins. The term is sometimes used to refer only to protein translation but more often it refers to a multi-step process, beginning with amino acid synthesis and transcription of nuclear DNA into messenger RNA, which is then...
in all living organisms. All prokaryotes have 70S (where S=Svedberg
Svedberg
A svedberg is a non-SI physical unit used for sedimentation coefficients. It characterizes the behaviour of a particle type in sedimentation processes, notably centrifugation. The svedberg is technically a measure of time, and is defined as exactly 10-13 seconds A svedberg (symbol S, sometimes...
units) ribosomes while eukaryotes contain larger 80S ribosome
Ribosome
A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
s in their cytosol
Cytosol
The cytosol or intracellular fluid is the liquid found inside cells, that is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into compartments....
. The 70S ribosome
Ribosome
A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
is made up of a 50S and 30S subunits. The 50S subunit contains the 23S and 5S rRNA while the 30S subunit contains the 16S rRNA. These rRNA molecules differ in size in eukaryotes and are complexed with a large number of ribosomal proteins, the number and type of which can vary slightly between organisms. While the ribosome
Ribosome
A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
is the most commonly observed intracellular multiprotein complex in bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
other large complexes do occur and can sometimes be seen using microscopy
Microscopy
Microscopy is the technical field of using microscopes to view samples and objects that cannot be seen with the unaided eye...
.
Intracellular membranes
While not typical of all bacteriaBacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
some microbes contain intracellular membranes in addition to (or as extensions of) their cytoplasmic membranes. An early idea was that bacteria might contain membrane folds termed mesosome
Mesosome
Mesosomes are folded invaginations in the plasma membrane of bacteria that are produced by the chemical fixation techniques used to prepare samples for electron microscopy...
s, but these were later shown to be artifacts produced by the chemicals used to prepare the cells for electron microscopy. Examples of bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
containing intracellular membranes are phototroph
Phototroph
Phototrophs are the organisms that carry out photosynthesis to acquire energy. They use the energy from sunlight to convert carbon dioxide and water into organic material to be utilized in cellular functions such as biosynthesis and respiration.Most phototrophs are autotrophs, also known as...
s, nitrifying bacteria
Nitrifying bacteria
Nitrifying bacteria are chemoautotrophic or chemolithotrophs depending on the genera bacteria that grow by consuming inorganic nitrogen compounds...
and methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
-oxidising bacteria. Intracellular membranes are also found in bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
belonging to the poorly studied Planctomycetes
Planctomycetes
Planctomycetes are a phylum of aquatic bacteria and are found in samples of brackish, and marine and fresh water. They reproduce by budding. In structure, the organisms of this group are ovoid and have a holdfast, called the stalk, at the nonreproductive end that helps them to attach to each other...
group, although these membranes more closely resemble organellar membranes in eukaryotes and are currently of unknown function.
Cytoskeleton
The prokaryotic cytoskeleton is the collective name for all structural filamentsProtein filament
In biology, a filament is a "long chain of proteins, such as those found in hair, muscle, or in flagella". They are often bundled together for strength and rigidity. Some cellular examples include:*Actin filaments*Microtubules*Intermediate filaments...
in prokaryotes. It was once thought that prokaryotic cells did not possess cytoskeleton
Cytoskeleton
The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...
s, but recent advances in visualization technology and structure determination have shown that filaments indeed exist in these cells. In fact, homologues
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
for all major cytoskeletal proteins in eukaryotes have been found in prokaryotes. Cytoskeletal elements play essential roles in cell division
Cell division
Cell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...
, protection, shape determination, and polarity determination in various prokaryotes.i
Nutrient storage structures
Most bacteriaBacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
do not live in environments that contain large amounts of nutrients at all times. To accommodate these transient levels of nutrients bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
contain several different methods of nutrient storage in times of plenty for use in times of want. For example, many bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
store excess carbon in the form of polyhydroxyalkanoates
Polyhydroxybutyrate
Polyhydroxybutyrate is a polyhydroxyalkanoate , a polymer belonging to the polyesters class that was first isolated and characterized in 1925 by French microbiologist Maurice Lemoigne. PHB is produced by microorganisms apparently in response to conditions of physiological stress...
or glycogen
Glycogen
Glycogen is a molecule that serves as the secondary long-term energy storage in animal and fungal cells, with the primary energy stores being held in adipose tissue...
. Some microbes store soluble nutrients such as nitrate
Nitrate
The nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...
in vacuole
Vacuole
A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain...
s. Sulfur is most often stored as elemental (S0) granules which can be deposited either intra- or extracellularly. Sulfur granules are especially common in bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
that use hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
as an electron source. Most of the above mentioned examples can be viewed using a microscope
Microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...
and are surrounded by a thin nonunit membrane to separate them from the cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
.
Gas vesicles
Gas vesicles are spindle-shaped structures found in some planktonPlankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
ic bacteria that provides buoyancy
Buoyancy
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...
to these cells by decreasing their overall cell density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
. Positive buoyancy is needed to keep the cells in the upper reaches of the water column, so that they can continue to perform photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
. They are made up of a shell of protein that has a highly hydrophobic inner surface, making it impermeable to water (and stopping water vapour from condensing inside) but permeable to most gases. Because the gas vesicle is a hollow cylinder, it is liable to collapse when the surrounding pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
becomes too great. Natural selection has fine tuned the structure of the gas vesicle to maximise its resistance to buckling
Buckling
In science, buckling is a mathematical instability, leading to a failure mode.Theoretically, buckling is caused by a bifurcation in the solution to the equations of static equilibrium...
, including an external strengthening protein, GvpC, rather like the green thread in a braided hosepipe. There is a simple relationship between the diameter of the gas vesicle and pressure at which it will collapse - the wider the gas vesicle the weaker it becomes. However, wider gas vesicles are more efficient, providing more buoyancy per unit of protein than narrow gas vesicles. Different species produce gas vesicle of different diameter, allowing them to colonise different depths of the water column (fast growing, highly competitive species with wide gas vesicles in the top most layers; slow growing, dark-adapted, species with strong narrow gas vesicles in the deeper layers). The diameter of the gas vesicle will also help determine which species survive in different bodies of water. Deep lakes that experience winter mixing, will exposes the cells to the hydrostatic pressure generated by the full water column. This will select for species with narrower, stronger gas vesicles.
The cell achieves its height in the water column by synthesising gas vesicles. As the cell rises up, it is able to increase its carbohydrate
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...
load through increased photosynthesis. Too high and the cell will suffer photobleaching and possible death, however, the carbohydrate produced during photosynthesis increases the cell's density, causing it to sink. The daily cycle of carbohydrate build-up from photosynthesis and carbohydrate catabolism
Catabolism
Catabolism is the set of metabolic pathways that break down molecules into smaller units and release energy. In catabolism, large molecules such as polysaccharides, lipids, nucleic acids and proteins are broken down into smaller units such as monosaccharides, fatty acids, nucleotides, and amino...
during dark hours is enough to fine tune the cell's position in the water column, bring it up toward the surface when its carbohydrate levels are low and it needs to photosynthesis, and allowing it to sink away from the harmful UV radiation when the cell's carbohydrate levels have been replenished. An extreme excess of carbohydrate causes a significant change in the internal pressure of the cell, which causes the gas vesicles to buckle and collapse and the cell to sink out.
Carboxysomes
Main article: CarboxysomeCarboxysome
Carboxysomes are bacterial microcompartments that contain enzymes involved in carbon fixation. Carboxysomes are made of polyhedral protein shells about 80 to 140 nanometres in diameter. These compartments are thought to concentrate carbon dioxide to overcome the inefficiency of RuBisCo - the...
Carboxysome
Carboxysome
Carboxysomes are bacterial microcompartments that contain enzymes involved in carbon fixation. Carboxysomes are made of polyhedral protein shells about 80 to 140 nanometres in diameter. These compartments are thought to concentrate carbon dioxide to overcome the inefficiency of RuBisCo - the...
s are intracellular structures found in many autotrophic bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
such as Cyanobacteria, Knallgasbacteria, Nitroso- and Nitrobacteria. They are proteinaceous structures resembling phage heads in their morphology
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
and contain the enzymes of carbon dioxide fixation in these organisms (especially ribulose bisphosphate carboxylase/oxygenase, RuBisCO, and carbonic anhydrase). It is thought that the high local concentration of the enzymes along with the fast conversion of bicarbonate to carbon dioxide by carbonic anhydrase allows faster and more efficient carbon dioxide fixation than possible inside the cytoplasm. Similar structures are known to harbor the coenzyme B12-containing glycerol dehydratase, the key enzyme of glycerol fermentation to 1,3-propanediol, in some Enterobacteriaceae (e. g. Salmonella).
Magnetosomes
Main article: MagnetosomeMagnetosome
Magnetosome chains are membranous prokaryotic organelles present in magnetotactic bacteria. They contain 15 to 20 magnetite crystals that together act like a compass needle to orient magnetotactic bacteria in geomagnetic fields, thereby simplifying their search for their preferred microaerophilic...
Magnetosome
Magnetosome
Magnetosome chains are membranous prokaryotic organelles present in magnetotactic bacteria. They contain 15 to 20 magnetite crystals that together act like a compass needle to orient magnetotactic bacteria in geomagnetic fields, thereby simplifying their search for their preferred microaerophilic...
s are intracellular organelles found in magnetotactic bacteria
Magnetotactic bacteria
Magnetotactic bacteria are a polyphyletic group of bacteria discovered by Richard P. Blakemore in 1975, that orient along the magnetic field lines of Earth's magnetic field. To perform this task, these bacteria have organelles called magnetosomes that contain magnetic crystals...
that allow them to sense and align themselves along a magnetic field (magnetotaxis). The ecological role of magnetotaxis is unknown but it is hypothesized to be involved in the determination of optimal oxygen concentrations. Magnetosome
Magnetosome
Magnetosome chains are membranous prokaryotic organelles present in magnetotactic bacteria. They contain 15 to 20 magnetite crystals that together act like a compass needle to orient magnetotactic bacteria in geomagnetic fields, thereby simplifying their search for their preferred microaerophilic...
s are composed of the mineral magnetite
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
or greigite and are surrounded by a lipid bilayer membrane. The morphology of magnetosome
Magnetosome
Magnetosome chains are membranous prokaryotic organelles present in magnetotactic bacteria. They contain 15 to 20 magnetite crystals that together act like a compass needle to orient magnetotactic bacteria in geomagnetic fields, thereby simplifying their search for their preferred microaerophilic...
s is species-specific.
Endospores
Main article: EndosporesPerhaps the most well known bacterial adaptation to stress is the formation of endospore
Endospore
An endospore is a dormant, tough, and temporarily non-reproductive structure produced by certain bacteria from the Firmicute phylum. The name "endospore" is suggestive of a spore or seed-like form , but it is not a true spore . It is a stripped-down, dormant form to which the bacterium can reduce...
s. Endospore
Endospore
An endospore is a dormant, tough, and temporarily non-reproductive structure produced by certain bacteria from the Firmicute phylum. The name "endospore" is suggestive of a spore or seed-like form , but it is not a true spore . It is a stripped-down, dormant form to which the bacterium can reduce...
s are bacterial survival structures that are highly resistant to many different types of chemical and environmental stresses and therefore enable the survival of bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
in environments that would be lethal for these cells in their normal vegetative form. It has been proposed that endospore
Endospore
An endospore is a dormant, tough, and temporarily non-reproductive structure produced by certain bacteria from the Firmicute phylum. The name "endospore" is suggestive of a spore or seed-like form , but it is not a true spore . It is a stripped-down, dormant form to which the bacterium can reduce...
formation has allowed for the survival of some bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
for hundreds of millions of years (e.g. in salt crystals) although these publications have been questioned. Endospore
Endospore
An endospore is a dormant, tough, and temporarily non-reproductive structure produced by certain bacteria from the Firmicute phylum. The name "endospore" is suggestive of a spore or seed-like form , but it is not a true spore . It is a stripped-down, dormant form to which the bacterium can reduce...
formation is limited to several genera of Gram-positive bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
such as Bacillus
Bacillus
Bacillus is a genus of Gram-positive, rod-shaped bacteria and a member of the division Firmicutes. Bacillus species can be obligate aerobes or facultative anaerobes, and test positive for the enzyme catalase. Ubiquitous in nature, Bacillus includes both free-living and pathogenic species...
and Clostridium
Clostridium
Clostridium is a genus of Gram-positive bacteria, belonging to the Firmicutes. They are obligate anaerobes capable of producing endospores. Individual cells are rod-shaped, which gives them their name, from the Greek kloster or spindle...
. It differs from reproductive spores in that only one spore is formed per cell resulting in no net gain in cell number upon endospore
Endospore
An endospore is a dormant, tough, and temporarily non-reproductive structure produced by certain bacteria from the Firmicute phylum. The name "endospore" is suggestive of a spore or seed-like form , but it is not a true spore . It is a stripped-down, dormant form to which the bacterium can reduce...
germination. The location of an endospore
Endospore
An endospore is a dormant, tough, and temporarily non-reproductive structure produced by certain bacteria from the Firmicute phylum. The name "endospore" is suggestive of a spore or seed-like form , but it is not a true spore . It is a stripped-down, dormant form to which the bacterium can reduce...
within a cell is species-specific and can be used to determine the identity of a bacterium.